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st: reg with xi and if exp


From   "Plattner, Dankwart" <dankwart.plattner@kfw.de>
To   "'statalist@hsphsun2.harvard.edu'" <statalist@hsphsun2.harvard.edu>
Subject   st: reg with xi and if exp
Date   Wed, 28 Aug 2002 17:31:02 +0200

Dear Co-Stata-Users,
I encounter a behavior I don't understand. I'm running a regression
(stripped down to its bones in order to better present my problem) with xi
and an if expression: 

. xi: reg qa_ek i.r2 if  r2>15
i.r2              _Ir2_15-93          (naturally coded; _Ir2_15 omitted)

      Source |       SS       df       MS              Number of obs =
85825
-------------+------------------------------           F(  2, 85822) =
132.51
       Model |  207587.248     2  103793.624           Prob > F      =
0.0000
    Residual |  67223628.8 85822  783.291333           R-squared     =
0.0031
-------------+------------------------------           Adj R-squared =
0.0031
       Total |  67431216.0 85824  785.691834           Root MSE      =
27.987

----------------------------------------------------------------------------
--
       qa_ek |      Coef.   Std. Err.      t    P>|t|     [95% Conf.
Interval]
-------------+--------------------------------------------------------------
--
     _Ir2_91 |  -4.046854   .2492706   -16.23   0.000    -4.535422
-3.558285
     _Ir2_92 |  (dropped)
     _Ir2_93 |  -2.513805   .2658109    -9.46   0.000    -3.034792
-1.992818
       _cons |   22.55939   .2058561   109.59   0.000     22.15592
22.96287
----------------------------------------------------------------------------
--

My questions are,
- why does STATA use r2==15 as the omitted category when I excluded this
category in the if exp? Is an if exp useless with xi when the same var is in
the if exp and handled by xi?
- why is r2=92 dropped? Is it because the omitted category was chosen
wrongly (there should be no with the data)?
Maybe somebody could possibly point me to an explanation and a remedy?
TIA
Sincerely 
Dankwart Plattner

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